Hex Head Bolts
What Are Hex Head Bolts?
Hex head bolts are externally threaded fasteners with a six-sided head, used with a nut or in a tapped hole. In common practice, “hex bolt” and “hex cap screw” are closely related; cap screws typically include a washer face and tighter dimensional tolerances, while standard hex bolts are often chosen where mechanical properties are the key requirement.
Types of Hex Head Bolts
- Hex bolts / hex cap screws: the standard choice for assemblies joined with a nut or tapped hole.
- Tap bolts (full-thread hex bolts): fully threaded for joints requiring thread along the entire grip.
- Flange bolts: hex head with an integrated flange that distributes load like a built-in washer.
- Heavy hex structural bolts: heavy-hex head, high-strength bolts for structural steel connections.
Materials & Grades
- Inch (SAE J429): Grade 2, Grade 5, Grade 8 (increasing strength). Head marks: none, 3 radial lines, 6 radial lines.
- Metric (ISO 898-1): property classes 8.8, 10.9, 12.9 indicate strength levels.
- Stainless steel: 18-8/304 for general corrosion resistance; 316 for marine or chemically aggressive settings.
- Finishes: zinc, hot-dip galvanized, black oxide, and others—select for environment and appearance.
Sizing & Threads
Specify size as diameter × thread × length (e.g., 1/2″-13 × 4″). Inch bolts commonly use UNC (coarse) or UNF (fine) threads. Coarse threads are standard for most field work; fine threads are used where greater adjustment or higher clamp loads are needed. Bolt length is measured from under the head to the tip.
Common Applications
- Construction and steel fabrication (frames, base plates, equipment mounts)
- Machinery and OEM assembly
- Automotive and equipment service
- Wood framing and general repair where nut-and-bolt joints are preferred
Installation Basics
- Match nuts and washers to the bolt’s grade/material (e.g., use compatible grades and coatings).
- Use proper torque methods; for structural bolting, follow the governing standard (e.g., turn-of-nut or calibrated wrench per project spec).
- If corrosion exposure is expected, choose stainless or a protective coating appropriate for the environment.
Related Hardware
- Hex nuts (matching grade/material and thread)
- Flat, fender, and lock washers
- Flange nuts (for flange bolts) and prevailing-torque nuts where specified
Hex Head Bolts FAQs
See the accordion below for differences between hex bolts and cap screws, grade selection, thread choice, and length measurement.
Both are hex-head fasteners. Cap screws typically include a washer face and tighter dimensional tolerances; hex bolts are often specified where the mechanical properties are the priority.
A fully threaded hex bolt. Choose it when the entire joint thickness needs thread engagement.
Inch-series: Grades 2, 5, and 8 (increasing strength). Structural heavy-hex bolts are specified to structural standards. Metric: property classes 8.8, 10.9, 12.9.
UNC (coarse) is the most common and more forgiving in field assembly. UNF (fine) is used in precision/high-strength joints with compatible nuts or tapped holes.
From under the head to the tip. (Countersunk styles are measured overall.)
Match grade/material whenever possible. Mixing can reduce joint strength or cause corrosion issues.
Standard hex bolts often use separate washers to spread load. Flange bolts integrate the washer under the head to speed assembly and increase bearing area.
No. “Grade 8” refers to quenched-and-tempered alloy-steel bolts. Stainless uses different designations (e.g., 18-8/304, 316).
Torque depends on diameter, thread, lubrication, and finish. Use the manufacturer’s table or project specification; for safety-critical joints, follow engineered procedures.
For outdoor or corrosive environments. Hot-dip galvanized provides a heavy zinc layer; stainless provides corrosion resistance without a coating. Choose based on environment, appearance, and cost.